http://sketchucation.com/pluginstore?pln=Pen_Tools
A suite of pen tools that allow to draw edges in a variety of ways that the native line tool doesn’t allow. Draw various edge type from soft edges to hidden edges and many more. Note: See ‘More Info’ on how to upgrade…

Well and good TIG, I’m sure plugins can make SketchUp do everything but mow the yard and swat flies.
But logically you must admit, drawing a hidden line shouldn’t require a tool switch. If the shift key works with the eraser, it should work with the line tool.

But of course the Shift key also locks inferences when drawing lines etc [and other modifier-keys do other things too], so it’s not so simple.
You can always draw ‘vanilla’ lines, using the native-tools, then select them, holding Ctrl to add to the selection etc, and use Entity Info to change their Hidden/Soft-Smooth-ness, or swipe them with the Erase tool+modifiers…

There are many things that the native-tools don’t do, plugins add functionality.
Your suggestion of having modifier keys to affect how the Line tool works interferes with the well-established modifier-key usage, so it’d be very confusing to most users.
On the relatively few occasions when you want to draw hidden or smooth edges, there are already several native workarounds to quickly modify ‘vanilla’ lines, and of course also Plugins when you need a more continual output…